1. Field of the Invention
This invention relates to zoom lenses of the rear focusing type, and more particularly to zoom lenses in which zooming is accomplished by moving three lens groups axially in differential relation and in which focusing is accomplished at the second or third group counting from front of the three lens groups.
2. Description of the Prior Art
Proposals have been made for a wide variety of zoom lenses which are focused at a part of the zoom section or a lens group which remains stationary during zooming in the rear of the zoom lens groups.
These focusing methods require one to differentiate between the amount of movement of the focusing section required for one object distance as the focal length changes with zooming, and the amount of focusing movement which varies with focal length along a complex curve or discontinuously. The complex curve changes in shape with the object distance. Therefore, an operating mechanism that allows the same angle of rotation of the distance adjusting ring to effect accurate focusing throughout the entire zooming range is of extremely complicated structure and in actual practice it is very difficult to realize. This tendency is increasingly intensified as the zoom ratio increases.
An example of a conventional four component zoom objective with the foregoing follows to show how a focusing movement changes as a different one of the four components is selected for focusing purposes. For this purpose, reference is made to FIGS. 1(a) to 1(e).
The zoom objective of FIG. 1(a) has four components I to IV of the following respective focal lengths fl to f4 with air separations l1 to l3 variable during zooming and with an object at infinity.
______________________________________ f1 110 f 80 144 200 f2 -40 l1 10 36.67 46 f3 111.167 l2 44.5 23.17 4.5 f4 121.273 l3 15 9.67 19 ______________________________________ f1-f4: focal length of individual lens groups l1-l3: principal point distance between individual lens groups
Using the negative-power second component II, the third component III movable for zooming, and the first component I for focusing is widely accepted in the prior art. Under these circumstances, while focusing down from infinity to an object at a given distance, the axial movement of first component I is maintained con stant at any location throughout the entire zoom range as illustrated in FIG. 1(b). But when the focusing is performed at the second component or those that follow, the axial movement becomes dependent upon the focal length of the entire system as illustrated in FIGS. 1(c) to 1(e).
The second component II represents a system that changes from a reducing one to an enlarging one during zooming, and passing unity of magnification on the way. The second component II is of negative power. Hence, in a reducing region the second component II must be moved forwards to focus down to shorter object distances. In the other region, for enlarging, it must be moved rearwards to effect an equivalent focusing result. In the transit for unit magnification focusing becomes uncertain, as illustrated in FIG. 1(c). Thus, the axial movement becomes discontinuous.
The first to third components in this example of the prior art form an afocal system. Hence when focusing independently with the third component the paraxial rays of light emerging from the third component the paraxial rays of light emerging from the third component become diverging as the object distance shortens from infinity. (At this time, this component becomes an enlarging system of positive sign.) To correct these diverging rays and make them parallel, the third component III may be moved rearwards. Thus focusing is effected, as illustrated in FIG. 1(d).
Similar to the case of the first component I, to use the fourth component IV for focusing, the latter may be moved forwards, because the rays incident on the fourth component IV are parallel, as illustrated in FIG. 1(e). However, the afocal magnification of the zoom section varies with zooming. Thus the focusing movement must be controlled as a function of the focal length. In this case, it has been found that the focusing movement of the fourth component IV is proportional to the square of the afocal magnification rate.
Also, as the object distance shortens, this effect becomes more severe as shown by the second degree curves for 5 and 2 meters in FIGS. 1(c) to 1(e).
In conclusion, to impart focusing movement to the second component or those that follow in the zoom objective of the aforesaid form, it is necessary to use one of several devices. One must use either a three-dimensional cam or other suitable control member that provides variations of the focusing movement not only as a function of the object distance but also as a function of the focal length of the entire system which can be represented by a continuous curve of secondary degree over the entire zoom range (when the angle of rotation of the distance adjusting ring is made to remain constant for the same object distance during zooming), or an electrically operated focusing control mechanism equipped with a computer circuit within the lens system or with a TTL auto-focus device.
A zoom objective having three lens groups movable for zooming is disclosed in U.S. Pat. No. 4,196,969. A zoom objective focusing at a lens group other than the first is described in U.S. Pat. No. 3,972,056.